1. Field of the Invention
The present invention relates to a cathode ray tube. More particularly, the present invention relates to a cost-effective cathode ray tube effective for minimizing deterioration of color purity and obtaining a sufficient margin for a beam strike neck (BSN) phenomenon by moving a ½ center (deflection center) closer to a panel without changing the curvature of a funnel or the thickness of a glass inside the funnel.
2. Background of the Related Art
FIG. 1 is a diagram explaining the structure of a generally known color cathode ray tube in a related art.
The color cathode ray tube has a fluorescent screen on a front surface of a cone-shaped vacuum tube, and there is an electron gun and a deflection yoke in a neck portion on the opposite side of the screen, whereby electron beams emitted from the electron gun are deflected and collided with the fluorescent screen to display an image.
As depicted in FIG. 1, a panel 1 and a funnel 2 of the color cathode ray tube are sealed up tightly together, so the inside of the cathode ray tube is generally in a vacuum state.
Speaking of the construction of the cathode ray tube, the fluorescent screen 3 with red (R), green (G) and blue (B) primary color phosphors (or fluorescent substances) is formed inside of the panel 1, and the electron gun 4 for emitting three color electron beams 7, namely red, green and blue, is installed in the neck portion of the funnel on the opposite side of the fluorescent screen 3.
A shadow mask 5 is disposed at a predetermined space between the fluorescent screen 3 and the electron gun 4, more specifically, closer to the fluorescent screen 3, for selecting colors. Also, in order to restrict the motion of the electron beams 7 promoted by a magnetic field, an inner shield 6, which is made of magnetic substance, is provided to a rear side of the brown tube to diminish an influence of a magnetic field thereon.
Meanwhile, there is a convergence purity correcting magnet (CPM) 8 around the neck portion of the funnel 2, which serves to adjust R, G and B electron beams emitted from the electron gun 4 to be converged to one single point, and in front of the magnet 8, there is a deflection yoke 9 for deflecting the electron beams 7.
In addition, a band 10 is put on the external skirt area of the panel, so as to reinforce a front surface glass with the presence of a high internal vacuum state (e.g. 10−7 Torr-10−8 Torr).
To briefly explain how the color cathode ray tube with the above construction operates, the electron beams 7 emitted from the electron gun 4 are deflected in the horizontal and vertical directions according to the deflection yoke 9, and the deflected electron beams 7 pass through a beam passing hole on the shadow mask 5 and eventually strike the fluorescent screen 3 on the front side, thereby displaying a desired color image.
Particularly, the CPM 8 corrects convergence and purity of R, G and B electron beams 7, and the inner shield 6, as it says, shields the rear cathode ray tube from the influence of the magnetic field.
As discussed before, the cathode ray tube is a high vacuum tube, meaning it is highly explosive by an external shock. For this reason, the panel is usually designed to be very strong enough to withstand atmospheric pressure.
Also, the band 10 put on the external skirt area of the panel 1 serves to disperse the tension on the high vacuum cathode ray tube, thereby providing the impact resistance to the tube.
FIG. 2 illustrates a cathode ray tube whose outer surface is substantially flat and inner surface has a predetermined curvature. Referring to the drawing, the cathode ray tube consists of a rectangular shaped panel 1 with a skirt area, the skirt area being vertically extended from the outer and inner surface, a funnel 2 coupled to a seal edge portion of the panel 1, a deflection yoke 9 for deflection electron beams, and an electron gun 4 for emitting electron beams. Particularly, FIG. 2 indicates that there is an area where a fluorescent screen inside the panel 1 gives little or no light. This phenomenon occurs because the electron beams deflected by the deflection yoke 9 strike the inner surface of the neck portion of the funnel, and they sometimes create an area that cannot radiate the screen mainly because the panel 1 nowadays is very light and slim.
In FIG. 2, reference numeral 1a suggests how big the panel used to be before it became much lighter; reference Ea shows the end of an effective surface of the screen from the old, heavy panel; reference numeral 1b suggests a panel after it became light; and reference Eb shows the end of an effective surface of the screen from the light panel.
Further, reference numeral 9a indicates a ferrite core; reference numeral 9b indicates an opening part of the deflection yoke; reference numeral 2a indicates a funnel curvature before and after the panel became light; and reference numeral 2b indicates a newly suggested funnel curvature to obtain a more margin of BSN.
Also, OAH (x) indicates the distance from the center on the outer surface of the panel to the center on an extended plane of a skirt seal edge part; and C indicates a deflection center, that is, a ½ center (or midpoint) of the ferrite core 9a. 
As illustrated in FIG. 2, from a mechanical sense, the deflection center of the deflection yoke 9 could be overlapped with the ½ center (C), on the ferrite core 9a. On the other hand, assuming that a coordinate axis (or reference line) exists around the ½ center (C), if an angle (α) between a vertical coordinate axis passing the ½ center on the ferrite core 9a and the end (Ea) of the effective surface of the screen from the old, heavy panel is determined, a margin for the funnel's deflection angle is created, and from there, a margin of beam strike neck is created also.
However, this does not happen to the light, slim panel 1 though. That is, if OAH (x) is reduced, the angle (β) between the vertical coordinate axis passing the ½ center on the ferrite core 9a and the end (Eb) of the effective surface of the screen from the light, slim panel 1 becomes greater than the angle (α). As a result thereof, the margin of the deflection angle of the funnel is decreased, and therefore, the margin of the beam strike neck is decreased as well.
Hence, in case of the light, slim panel 1, the electron beams deflected by the deflection yoke 9 are collided with the neck portion of the funnel 2, and this actually creates an area on the effective surface of the screen, where no electron beams emits light.
This BSN phenomenon consequently gives rise to another problem, such as, degraded color purity.
In order to overcome the above problems and obtain more BSN margin, a number of attempts have been made. For instance, some tried to redesign the curvature of the funnel (i.e. from 2a to 2b), or make the thickness of a glass inside of the funnel 2 thinner. The thing was that it cost too much time, efforts, and expenses. On the top of that, the depth of the funnel was prolonged in the process of redesigning the curvature of the funnel.